Conventional humidity measurement is typically done optically using infrared detection systems. Such systems are large and employ continuous infrared radiation sources. Because of their relatively large thermal inertia, such infrared sources cannot be effectively pulsed and instead mechanical choppers must be used to provide the modulated signals required by the devices. Additionally, conventional infrared detectors are sensitive to background heat which includes infrared radiation.
Ultraviolet humidity detectors have been provided which employ a Lyman-Alpha hydrogen source. One such device dissociates water into its constitutent OH radicals and hydrogen atoms. The emission spectra of the OH radicals are measured to indicate the amount of water present in the atmosphere. Another such device monitors the reduced transmission of the Lyman-Alpha source emission directly to determine humidity. These systems use continuous UV sources which are not extremely reliable and have a limited life. Therefore they are generally not viable for commercial use and have been employed primarily for research.
There is an ultraviolet oxygen (O.sub.2) detector which employs two very close emission isotope lines from a continuous source. One of these lines is absorbed by O.sub.2 ; the other is not. However, the water absorption at each of these wavelengths is virtually the same and therefore the system is impractical for measuring humidity.